1. Field of the Invention
The present invention relates to a method and an apparatus for optical surface analysis of a sample area on a sensor surface.
2. Description of the Related Art
The interest for surface sensitive measuring techniques has increased markedly recently as several optical techniques have been developed for identifying and quantifying molecular interactions, which techniques do not require labelling. The most used optical technique so far is based on surface plasmon resonance, hereinafter frequently referred to as SPR.
The phenomenon of surface plasmon resonance, or SPR, is well known. In brief, SPR is observed as a dip in intensity of light for a specific wavelength reflected at a specific angle (as measured by, e.g., a photodetector) from the interface between an optically transparent material, e.g., glass, and a thin metal film, usually silver or gold, and depends on among other factors the refractive index of the medium (e.g., a sample solution) close to the metal surface. A change of the real part of the complex refractive index at the metal surface, such as by the adsorption or binding of material thereto, will cause a corresponding shift in the angle at which SPR occurs, the so-called SPR-angle. For a specific angle of incidence, the SPR is observed as a dip in intensity of light at a specific wavelength, a change in the real part of the refractive index causing a corresponding shift in the wavelength at which SPR occurs.
To couple the light to the interface such that SPR arises, three alternative arrangements may be used, viz., either a metallized diffraction grating (see H. Raether in “Surface Polaritons”, Eds. Agranovich and Mills, North Holland Publ. Comp., Amsterdam, 1982), a metallized glass prism (Kretschmann configuration) or a prism in close contact with a metallized surface on a glass substrate (Otto configuration). In a SPR-based assay, for example, a ligand is bound to the metal surface, and the interaction of this sensing surface with an analyte in a solution in contact with the surface is monitored.
Originally, collimated light was used for measuring the SPR-angle, the sensing area being restricted to the intersection of the collimated light beam and the metal surface. The apparatus used was based on mechanical goniometry with two movable mechanical axes carrying the illumination and detecting components, the rotational center of which was placed in the center of the sensor area, i.e., one axis for the incident light and another axis for the reflected light which was detected by a single photodetector. A plane-sided coupling prism at total internal reflection condition was used to preserve the collimated beam inside the prism, which, however, introduced a refraction at non-orthogonal incidence at the prism, thereby introducing a beam-walk at the metal surface. A half-cylindrical coupling lens with orthogonal incidence of the optical axis of the light gave a fixed sensor area, however, introduced a beam-convergency, i.e., a non-collimated or quasi-collimated beam inside the prism.
In a development, the movable opto-mechanical axis on the illumination side was eliminated by using a focused incident beam, so-called focused attenuated total reflection (focused ATR), as described by Kretschmann, Optics Comm. 26 (1978) 41–44, the whole angular range simultaneously illuminating a focal line or point of a given sensing surface. The use of a detector matrix for detecting the reflected light eliminated the movable opto-mechanical axis also on the reflectance side, providing a faster SPR-detection than that of the prior art. Such systems are described in, e.g., EP-A-305 109 and WO 90/05295. In the latter, light beams reflected from a specific sub-zone at the sensor area are imaged anamorphically so that beams in one plane (the sagittal plane) create a real image on a specific detector-pixel row of a matrix detector, permitting the occurrence of a local surface binding reaction to be identified, while quantification of the reaction is obtained via angular data for a reflectance curve measured along the same pixel row, where the reflectance curve is created by beams in a plane (the meridian plane) normal to the first-mentioned plane. Thus, one dimension of the matrix detector is used for real imaging simultaneously as the other dimension is used for only angular measurement. This permits only sub-zones arranged in a row to be simultaneously monitored and imaged.
In a variation of goniometry, the bulky mechanical axes were replaced by rotating or vibrating mirrors, respectively. A disadvantage of that approach is, however, that when scanning the incident angle by means of a plane mirror, the point where the light beam hits the sensor surface will move along the internal reflection surface of the prism. This problem was avoided by using a combination of rotating mirror and focused SPR, e.g., as described by Oda, K., Optics Comm. 59 (1986) 361. In this construction, a first collimated beam of about 1 mm diameter impinges on the rotational center of a rotating mirror placed at the focal length of a focusing lens, thus producing a second quasi-collimated beam, the distance of which to the optical axis depends on the reflecting angle of the mirror. The second collimated beam is focused by a second focusing lens onto a prism base at total internal reflection conditions. During the rotation of the mirror, the angle of incidence at the approximately fixed sensor area is scanned for the quasi-collimated beam.
Another approach to obtain a fixed and also enlarged sample spot is proposed by Lenferink et al., “An improved optical method for surface plasmon resonance experiments”, vol. B3 (1991) 261–265. This technique uses the combination of a collimated light beam illuminating a plane rotating mirror, a focusing cylindrical (convex) lens after the mirror and a half-cylinder lens for coupling the light to the sensing surface. By making the cylindrical lens focus the light on the focal surface of the coupling half-cylinder, using a relatively complex lens system, a collimated beam is obtained inside the coupling prism.
Other optical techniques similar to SPR are Brewster angle reflectometry (BAR) and critical angle reflectometry (CAR).
When light is incident at the boundary between two different transparent dielectric media, from the higher to the lower refractive index medium, the internal reflectance varies with angle of incidence for both the s- and p-polarized components. The reflected s-polarized component increases with the angle of incidence, and the p-polarized component shows a minimum reflectance at a specific angle, the Brewster angle. The angle at which both s- and p-polarized light is totally internally reflected is defined as the critical angle. For all angles of incidence greater than the critical angle, total internal reflection (TIR) occurs.
Schaaf et al., Langmuir, vol. 3 (1987) describes Brewster angle reflectometry using a micro-controlled rotation table and a movable detector in scanning angle reflectometry around the internal Brewster angle to study a protein (fibrinogen) at a silica/solution interface. The use of movable optomechanical axes and a rotation table gives a slow measuring procedure, and the sensor area is restricted to the cross-section of the collimated light beam with the sensor surface being limited by the need for non-beam-walking.
A focusing critical angle refractometer, based on a wedge of incident light which strikes the line of measurement, including the critical angular interval to be measured, and which measures the one-dimensional refractive index profile along a focused line immediately adjacent to the glass wall of a liquid container is described by Beach, K. W., et al., “A one-dimensional focusing critical angle refractometer for mass transfer studies” Rev. Sci. Instrum., vol. 43, 1972. This technique is limited in that it enables only a one-dimensional sensor area, which is restricted to the cross-section of the light line with the sensor area.
In all the above described prior art methods for reflectometric measurements, the reflectometric signal obtained represents an average value for the sensor surface and the size of the sensor surface is restricted, or minimized, to the extension of the collimated or quasi-collimated narrow beam, or focusing point or line. Therefore, such SPR-based methods used to measure, for example, inter alia protein interactions are limited to quantitative information for sensing areas localized in one spot or one row of spots on the surface where a specific interaction takes place. Approaches to monitor a two-dimensional interaction pattern have been made for both macroscopic and microscopic SPR-based imaging of a sensing surface.
Thus, EP-A-341 928 discloses a method for monitoring a large SPR sensor area in real-time by scanning a small focused beam, of, e.g., 10 μm, as a measuring sensing surface successively over the large area, more specifically a DNA sequencing gel, for example 20×20 cm2, thereby making it possible to build up an image or picture of the sample distribution within the sequencing gel by means of a photodetector array. This method requires, however, the use of scanning mirrors for both addressing the sensor zones and scanning the angle and complex and expensive processing of angular and positional data from the mirror-scanners and photodetectors, which limits the detection rate.
Yeatman and Ash, Electronic Letters 23 (1987) 1091–1092, and SPIE 897 (1988) 100–107 disclose microscopic real imaging of the sensing surface, so called surface plasmon microscopy, or SPM. This was achieved by the use of SPR in the Kretschmann configuration with a triangular prism for imaging dielectric patterns deposited on a silver film with a lateral resolution of about 25 μm, utilizing a focused beam in the form of a line scanned along the sensor surface. Also described is the use of an expanded laser beam at the resonance angle to illuminate a larger area and make a photograph of such an image by a positive lens in front of the photodetector. The illuminating beam is collimated and the angle of incidence is adjusted by rotating the triangular prism. The method is proposed to be used for the examination of metal films, biological and other superimposed monolayers.
Image processing methods for such SPM using a collimated beam and a lens inserted into the reflected beam to create an image of the sample distribution at the prism base are discussed in by Yeatman and Ash in “Computerized Surface Plasmon Microscopy”, SPIE, Vol. 1028 (1988) 231.
Okamoto and Yamaguchi have described a SPR-microscope wherein the position of a collimated beam, SPIE vol. 1319 (1990) 472–473, or a focused beam, Optics Communications 93 (1992) 265–270, is scanned across the sample surface in a Kretschmann configuration, the assembled point-SPR data thereby creating an image. In the focused beam alternative, a linear photodiode array is used for detection of the SPR-angle, in accordance with the principle of focused ATR as described earlier by Kretschmann, supra.
Drawbacks with mechanically scanned SPR-sampling includes that a high lateral resolution and high speed for “real time monitoring” demands a complex and expensive scanning mechanics (due to the bulkiness of the illumination and detection device).
EP-A-469377 describes an analytical system and method for the determination of an analyte in a liquid sample based on surface plasmon imaging. Surface plasmon images as a function of the angle of incidence are monitored by a CCD-camera and analyzed by an image-software. Algorithms are used for comparing the measured SPR-angle for different areas of the sensor surface for the purpose of eliminating the contribution from a non-specific binding, and of calibration the response curve.
Rothenhausler and Knoll, Nature, 332 (1988) 615–617, demonstrates surface plasmon microscopy on organic films (a multilayer cadmium arachidate), based on SPR in a Kretschmann configuration. A simple lens is used to form an image of the sample/metal interface. A collimated beam (plane waves) illumination and a movable bulky optomechanical axis are used to change the angle of incidence.
In the above prior art arrangements for SPM, the angle of incidence is varied by the use of goniometry, either in a form where one or both optomechanical axes are moved (scanned) in relation to a fixed prism, or in a form using a movable optomechanical axis in combination with a rotating prism, the common rotation point being in the center of the prism sensor surface, and the angle of incidence being derived from the change in position or rotation of the mechanical axis through mechanical, electronic, electromagnetic or optical means. It is readily understood that such variation of the incident angle based on rotation of the mechanical axis carrying illuminating, imaging and detector modules is rather slow and inaccurate if not a complex and expensive design is provided.
Another approach is disclosed by Kooyman and Krull, Langmuir 7 (1991) 1506–1504, namely SPM using a small vibrating mirror in combination with a Kretschmann configuration to adjust the angle of incidence. A disadvantage is, however, that the probed spot is not stationary during the angular scan, i.e., all sites within a sensing area are not probed by light at an equal angle of incidence range, unless bulky optics is covering the sensing area at an excess.
Also microscopy based on Brewster angle measurements has been described. Hènon and Meunier, Rev. Sci. Instrum., vol. 62 (1978) 936–939 discloses the use of a microscope at the Brewster angle for direct observation of first-order phase transitions in monolayers. In this case, external Brewster angle is measured, i.e., no internal reflection and no coupling prism, the camera plane being parallel with the sensor surface.
Similarly, Hönig and Mobius, J. Phys. Chem., vol. 95 (1991) 4590–4592 describes the use of Brewster angle microscopy to study the air-water interface. Objects of about 3 μm diameter could be visualized by video recording of p-polarized light reflected under a fixed Brewster angle for the pure water surface.
A microscopic imaging ellipsometer has been described by Beaglehole, D., Rev. Sci. Instrum., 59 (12) (1988) 2557–2559.
Multiple-angle evanescent wave ellipsometry, in the form of using rotating optical means and a rotating prism for the variation of incident angle, and a phase-modulated ellipsometer, has been used for studying the polymer (polystyrene) concentration profile near a prism/liquid interface; see Kim, M. W., Macromolecules, 22, (1989) 2682–2685. Furthermore, total internal reflection ellipsometry in the form of stationary optical means at a single angle of incidence has been suggested for quantification of immunological reactions; see EP-A1-O 0 67 921 (1981), and EP-A1-0 278 577 (1988).
Azzam, R. M. A., Surface Science 56 (1976) 126–133, describes a use of evanescent wave ellipsometry, wherein both the intensity and polarization ellipse of the reflected beam can be monitored as functions of the angle of incidence, wavelength or time. Under steady state conditions, measurements as a function of wavelength and angle of incidence can provide basic information on the molecular composition and organization of the (biological) cell periphery. In a dynamic time-varying situation, measurements as a function of time can resolve the kinetics of certain surface changes.
Abelès, F. et al., in Polaritons, Editor E. Burstein and F. De Martini, Pergamon Press, Inc., New York, 1974, 241–246, shows how extremely sensitive surface plasmons are to very fine modifications of the surface, and the advantage of then measuring not only the reflected amplitude, but also the ellipsometric parameters, amplitude and phase, of the reflected wave.
A focusing critical angle refractometer for measuring a one-dimensional refractive index profile being displayed in a graphic image for mass transfer studies has been described by Beach, K. W., The Review of Scientific Instruments, 43, No. 6 (1972) 925–928. Since this apparatus uses one dimension in the image plane for a real image, and the other dimension for projecting the reflectance versus angle of incidence, it could not, however, provide a two-dimensional image of the refractive index distribution.
The prior art microscopy systems described above do not permit a sufficiently rapid, sensitive and accurate scanning and measurement of the incident angle to permit highly quantitative multi-site real-time monitoring of a sensor surface. Further, they are only suitable for imaging a limited sensor area of up to about 1×1 mm2. They are also too laborative and operator-dependent to be used in a commercial analytical instrument.
More particularly, the prior art apparatuses and systems for SPR microscopy and Brewster angle microscopy use either a high inertia scanner in the form of a goniometer where one or both mechanical axes for the illumination side and the imaging side, respectively, are rotated in relation to a fixed prism, or a movable mechanical axis in combination with a rotating prism for scanning the optical axis, i.e., the angle of incidence. In case the illumination consists of a collimated beam of light, the incident angle is measured directly on the angle steering signal without using the incident light, e.g., on the control signal to the rotor-driving motor of a mirror, or by an electronic or optoelectronic angle sensor placed on or at the rotated axis. Alternatively, a galvanometric or resonant low inertia scanner is used to drive a vibrating mirror to oscillate within a given calibrated angular range, determined by a given drive current, without monitoring the actual incident angle of the light.
The necessary high refractometric sensitivity for determining the refractive index of the sensor surface with an apparatus constructed according to the above prior art in a long-term accurate commercial analytical instrument could therefore only be achieved with a very complex and expensive design.